In Xenopus laevis, vitellogenin which is synthesized in the liver under control of estrogen is encoded in a small family of genes. Two of the vitellogenin genes together with long stretches of their flanking regions have been isolated from a Xenopus gene library. The structural organization of these two genes has been determined by electron microscopy. In both genes, the mRNA coding sequence is interrupted 33 times by sequences (introns) not present in mature vitellogenin mRNA.